(1) Field of the Invention
The present invention relates to an image forming apparatus such as a printer, copier or the like, which forms an image on a sheet of paper, in particular, relating to an image forming apparatus which uses an optical sensor that detects the density and misregistration of the image in order to assure image quality.
(2) Description of the Prior Art
In image forming apparatuses based on electrophotography, a test pattern image (reference toner image) for checking whether the image forming process is executed correctly is often used. Typical examples of test pattern images include the toner image for density measurement and the toner image for positional check.
This test of the image forming process is carried out by forming a reference toner image on a trial basis, sensing the density and position of the thus formed reference toner image with an optical sensor and detecting whether a toner image having a correct density is formed at an exact position. In this test, if the conditions of the image forming process are not pertinent, the image forming process conditions are corrected in accordance with the detected result of the optical sensor. In this way, this test using a reference toner image is periodically implemented, whereby it is possible to achieve image forming operations under pertinent conditions.
In color image forming apparatuses, it is impossible to reproduce a faithful color image unless images of color separations are laid over in register with each other. To avoid this, the above-described detection, or check on whether there is any problem with toner density and image position, is periodically implemented using an optical sensor with a reference toner image.
However, the optical sensors used for toner density sensing and registration sensing have to be arranged at positions where unfixed toner passes through. Besides, the aforementioned reference toner image is formed on the transfer belt and detected by the optical sensors, and in order to achieve reliable detection, the sensors have to be positioned at the most downstream side in the image forming process.
Accordingly, quite a few amount of unfixed toner scatters when the recording medium (paper) is transferred from the toner belt to the fusing device. As a result, the unfixed toner gradually builds up on the detection surface of the sensors, possibly causing detection deterioration and detection failure.
To deal with this problem, patent document 1: Japanese Patent Application Laid-open 2001-100597, discloses a configuration in which a shutter is provided in the vicinity of the detecting surface of an optical sensor in an openable and closable manner so that the shutter will open to permit detection only for the time of measurement and can close itself otherwise, to thereby prevent adherence of unfixed toner. Further, in patent document 1, the optical sensor for detecting the light reflected off the reference toner image is made active during the opening and closing movement of the shutter so as to be used for detecting the opening and closing of the shutter by detecting the reflected light from the transfer belt. That is, check on whether the shutter is correctly opened and closed is enabled without providing a separate sensor for detecting the opening and closing of the shutter.
However, in the case having the configuration as in patent document 1, if the toner has adhered on the transfer belt when the shutter changes its position from the close state (no light of direct reflection received) to the open state (light of direct reflection received), light, emitted from the optical sensor and reflected off the toner layer, shifts out of the light path of direct reflection from the transfer belt due to the thickness of the adhered toner layer, hence the optical sensor can receive no light of direct reflection (no light of direct reflection received) and detects the shutter being closed in error even though the shutter is opened correctly.
Then, when the toner area on the transfer belt has moved away from the shutter, the optical sensor finally receives light of direct reflection and detects the shutter being open. However, at this timing, the offset cam for driving the shutter has already rotated past and gotten out of the position, causing the problem that the open/close status of the shutter becomes unstable.